The present invention relates generally to the problem of metal and metal alloy workpieces absorbing hydrogen when undergoing heat treatment in furnaces containing ambient moisture-laden atmospheres, and particularly to transition metal salt compositions that substantially reduce the absorption of hydrogen into such workpieces and, in addition, greatly enhances hydrogen degassing of such workpieces. The control of bulk hydrogen levels can be critical to the mechanical reliability of products fabricated from commercially significant metals or alloys, containing aluminum, nickel, tantalum, titanium, copper, iron, zirconium and magnesium.
When a metal or metal alloy object is heated in the presence of moist air, a protective oxide layer on the object is invariably disrupted to expose nascent metal. Aluminum oxidation in the presence of water, for example, while in a heated furnace, generates atomic hydrogen, which readily diffuses into the aluminum object, and is the only gas that has appreciable solubility in aluminum. Still, atomic hydrogen has limited solubility in metal and has the propensity to precipitate in the metal as insoluble molecular hydrogen (H.sub.2) at heterogeneities or defects, especially in highly worked regions within the metal object. As increasing hydrogen is precipitated and trapped within the metal, additional hydrogen can be absorbed and solubilized within the metal matrix. Bulk porosity in a metal workpiece, including porosity that is induced or enhanced by precipitated molecular hydrogen, can compromise structural integrity and the mechanical performance of the final metal parts.
For several decades, ammonium fluoborate (NH.sub.4 BF.sub.4) protective atmospheres have been used in the industry to prevent substantial absorption of hydrogen by aluminum alloy workpieces during high temperature furnace treatments. Ammonium fluoborate decomposes during such treatments at temperatures above 482.degree. F. to form a blanket atmosphere that fills the entire internal volume of a furnace. Ammonium fluoborate also produces an array of compounds in the furnace which can eliminate high temperature oxidation reactions by either reacting with ambient water or by forming a protective fluorinated layer on the aluminum alloy workpiece.
There are drawbacks to the use of ammonium fluoborate atmospheres, however. Ammonium fluoborate species can stain and pit surfaces of some aluminum alloys. The ammonium fluoborate decomposition products contain toxic, corrosive and particulate species. The ammonium fluoborate emissions corrode furnace structures and baghouses for filtering particulate emissions. Disposal of the collected particulates is costly. Concerns relating to the emissions have prompted research to identify alternative chemistries that are more environmentally friendly and safer for in-plant use.